This invention pertains in general to a wireless communications system, and, more particularly, to a method and apparatus for a sub-system search window placement and size determination for a wireless communications system.
In a typical CDMA or WCDMA wireless communications system, a transmitted signal travels from a transmitter to a receiver over multiple paths. Prior to transmission, a base station multiplies the information signal intended for each of the mobile stations by a unique signature sequence, referred to as a pseudo-noise (PN) sequence. The signals for all subscriber mobile stations are then transmitted simultaneously by the base station. Upon receipt, each mobile station demodulates the received signal, the result of which is integrated to isolate the information signal intended for a particular mobile station from the other signals intended for other mobile stations. The signals intended for the other mobile stations appear as noise.
During transmission, each multi-path is considered a separate channel subject to interference effects such as fading, dispersion and birth-death. The CDMA or a WCDMA system employs a “rake” receiver, which demodulates a received signal using plural demodulation “fingers”, each of which demodulates a signal component from a number of the channel paths. A typical rake receiver includes a plurality, from three to six, rake branches or “fingers,” each of which is an independent receiver unit that assembles and demodulates one received multi-path assigned to the finger. The outputs of the rake fingers are combined to improve performance. Before the multi-path signals are demodulated, however, the delays of the multi-path signals must be ascertained.
To ascertain multi-path signal delays, a rake receiver operates in conjunction with a delay searcher and a plurality of delay trackers. The delay searcher conducts a “coarse” searching with a rough resolution so as to quickly analyze a received signal and ascertain the delays, which are then assigned to the rake fingers. In mobile communications, the channels may be subject to additional fading due to the motion of the receiver. The delay trackers therefore track the delays assigned by the searcher between channel searches. Thus, while the searcher looks over a wide range of delays, the trackers look for a smaller range surrounding the assigned delays.
Specifically, the delay searcher conducts searches within a predetermined search window at a given resolution. FIG. 1 shows a general representation of a search window and search resolution. Within each search window, there are a number of candidate paths, expanding from the shortest path to the longest path. Depending upon the size (width) of the search window, hundreds and even thousands searches for candidate paths may be required. An example of a searcher is a Multiple-Path Searcher (MPS) implemented in the wireless receiving end. It is preferable to reduce the search for true location of the path after sufficient path information is acquired. Therefore, a window decision mechanism would determine window placement and size of the search window.
However, search windows are generally provided by the network and are optimized only for handoff performance. Consequently, the estimator will typically exceed available search time constraints if the size of the predetermined search window is set too large or the window is inappropriately placed. One prior art proposes adjusting the window placement either to the right or left of the window center where the maximum power path identified. The adjustment is accomplished by increasing or decreasing a counter that controls the window placement.
Another prior art proposes a mechanism for window placement that configures forward and backward boundaries based on a reference power path delay or search window boundary. Window placement is adjusted when searched paths exceed the boundaries. The referenced power path delay may be set as the location of a reference power path delay from a previous adjustment, and the boundaries are set at equidistant distances from the reference.
U.S. Pat. No. 6,775,252 by Bayley describes a method and apparatus of adjusting a search window size by a remote unit in a slotted mode wireless communication system. Bayley describes that in a slotted mode communication system, the remote unit is in an “active state” during its assigned slot, and the controller in a remote unit passes selected sets of search parameters to a search engine. The search engine performs searches on base stations using the selected sets of search parameters. One search parameter, the search window size, is adjusted in response to a measured signal strength of a first base station signal. The adjusted search window size is used by the remote unit when searching other base stations. A finite state machine of the mechanism of Bayley, originally disclosed as FIG. 8, is reproduced as FIG. 2.
U.S. Pat. No. 6,738,438 by Rick et al. describes a parameter estimator for estimating one or more parameters from a correlation function derived from a signal using a dynamically variable search window. According to Rick et al., the parameter estimator may be employed in a subscriber station to estimate the time of arrival of one or more base station pilot signals in a wireless communication system. Therefore, Rick et al. appears to describe a method and mechanism that adjust window placement and window size at the same time, depending upon different thresholds to compare with the maximum power path received in order to ascertain the placement of the search window.